This study is primarily designed to determine beta-haplotypes and RBC deformability on our current patients with sickle cell anemia. A major specific objective is to determine if the beta haplotype (Benin, CAR, Senegal) has an effect on RBC deformability or not. In addition the alpha-gene number and Hb F level will also be determined. Another major objective is to determine the rheological properties of sickle RBC in the steady state and during painful crises in patients with different beta-- haplotypes and alpha-gene number. A third objective is to determine the effect of hydroxyurea on the rheological properties of sickle erythrocytes in vivo. This part of the study will be a parallel project to the Multicenter Study of Hydroxyurea (HU) in Sickle Cell Anemia. Genomic DNA will be prepared from peripheral white blood cells from patients with sickle cell anemia. The alpha-gene number and beta-haplotypes will be determined by the Southern blotting technique or by polymerase chain reaction (PCR). Rheological properties of RBC will include: 1) RBC deformability determined by Ektacytometry; 2) number of dense cells determined by centrifugation on discontinuous Stractan density gradient and 3) RBC cations (Na+ and K+) determined by flame photometry. The rheological studies will be performed on unfractionated cells as well as on density-defined cells. It is projected that at least 15 of our patients will enroll in the HU study. Multiple base line studies (at least three determined at monthly intervals) will be performed before the initiation of drug therapy. Rheological studies will be done monthly after the initiation of drug therapy for the period of the study and for a minimum of four months after the discontinuation of drug therapy. In addition, survival of 51Cr-labeled autologous erythrocytes, 51Fe uptake by the bone marrow, plasma iron turnover and iron utilization will also be determined. These erythrokinetic studies will be done before the initiation of Hydroxyurea/placebo therapy and every six months thereafter. They will also be determined 4-6 months after the discontinuation of drug therapy. It must be emphasized that during the course of the study the principal investigator will have no access to any data such as the MCV, deformability index, etc. which may break the double-blind nature of the study. We hope this study will generate data which will: 1) clarify the factors which affect the clinical phenotype of sickle cell anemia; 2) elucidate the pathophysiologic changes that occur during the painful sickle cell crisis; and 3) provide insight into the mechanism of action of HU and its cellular effects which are not secondary to the increased level of Hb F. Moreover, determination of the rheological properties of RBC may be of value in monitoring the response to HU.